This invention is related to an initial core loaded before operation of a nuclear reactor. A nuclear reactor requires continuous operation for a fixed period without supplying fuel, and the core contains a larger quantity of fissionable materials than necessary to maintain the critical state of the nuclear reactor. Therefore, if there is no control material in the core, a critical excess in reactivity will result. This excessive in reactivity is called excess reactivity, and it is important to control the excess reactivity properly throughout the operation cycle. As a technique to do so, it is well known to mix a burnable poison in the fuel. The burnable poison is a neutron absorber that burns gradually during the operation cycle, and gadolinia is a typical known burnable poison.
Next, the suppression effect of the reactivity by the burnable poison will be explained by reference to FIG. 3. FIG. 3 shows a relation between the infinite multiplication factor of a fuel assembly containing gadolinia as a kind of the burnable poison and the exposure. As is shown in FIG. 3, as the number of fuel rods with gadolinia (Gd fuel rods) is reduced, the infinite multiplication factor at the early stage of the exposure increases. On the other hand, if the density of gadolinia (density of Gd) is increased, the maximum value of the infinite multiplication factor can be suppressed, because the time when the gadolinia will burn out can be delayed. Therefore, excess reactivity can be properly controlled by a selected combination of the density of the burnable poison and the number of the fuel rods with the burnable poison.
Next, the improvement of the fuel economy of the initial core will be explained. Parts of fuel assemblies loaded into the initial core are taken out after the first operation cycle (first cycle) and these are exchanged for reload fuel assemblies. The fuel assemblies taken out after the first cycle have a lower burnup and a lower generated energy than other fuel assemblies. Then, to efficiently utilize the fissionable materials, the formation of an initial core using a plurality of fuel assemblies that have a different uranium enrichment according to the duration of their use in the reactor is determined.
With regard to this initial core, a core, composed of high enrichment fuel assemblies with 3.4 wt % average enrichment, middle enrichment in the axial direction of fuel assemblies with 2.3 wt % in the axial direction of the fuel assemblies and low enrichment fuel assemblies with 1.1 wt % in the axial direction of fuel assemblies, has been described in Japanese Patent Laid-open print No. 5-249270. It is also described in this publication that fuel assemblies with lower average enrichment in the axial direction are taken out from the core in an earlier stage and other fuel assemblies with a higher average enrichment in the axial direction are loaded into the core for a long period to efficiently utilize the fissionable materials.
With regard to a known way of improving the fuel economy of the initial core, it has been described that fuel assemblies having a higher average enrichment than reload fuel assemblies are loaded in the most outer position of the core in Japanese Patent Laid-open print No. 60-13283.
Another known way to improve the fuel economy of the initial core is described in Japanese Patent Laid-open print No. 61-165682. In this publication, an increase of exposure which originated from the start-up test is compensated by increasing the number of the high enrichment fuel assemblies of the initial core to more than the number of reload fuel assemblies of the equilibrium core. As a result, the fuel economy of the initial core is improved. In this known arrangement, the enrichment of the high enrichment fuel assembly is the same as that of the reload fuel assembly.
The nuclear reactor requires operation with a proper control of the reactivity for a constant term. Generally, the increase of excess reactivity caused by increasing the average enrichment of the initial core in the axial direction is reduced by insertion of control rods or by mixing burnable poison into the fuel. But, in a core which is provided with an increased average enrichment in the axial direction for purposes of obtaining a higher burnup, the excess reactivity also increases further. When the control rods are inserted to reduce the increase of the excess reactivity, with an increase in the number of the inserted control rods, the channel peaking factor increases, and the thermal margin decreases. Moreover, it is also necessary to repeat an adjustment of the quantity of the control rods being inserted to compensate for a large change of the excess reactivity in the operation cycle. This reduces the availability factor of the nuclear reactor, and it is not desirable from the viewpoint of fuel economy. In case of increasing the mixing quantity (density) of the burnable poison, while the excess reactivity could be suppressed by increasing the mixing quantity (i.e. enrichment) of the burnable poison, this causes the thermal conductivity of the fuel pellets to decline, producing a problem from the integrity point of view.
As is mentioned above, an increase of the excess reactivity of the core was the main factor for a disturbance in case of achieving a high burnup of the initial core.
When the high burnup of the initial core is designed according to the technique described in Japanese Patent Laid-open print No. 60-13283, a sufficient effect is not achieved. In this case, because fuel assemblies with a high average enrichment in the axial direction are only loaded into the most outer position of the core, the number of the fuel assemblies with a high average enrichment in the axial direction is limited. Consequently, there is a limit to the improvement in the fuel economy to be obtained by increasing the average enrichment in the axial direction of the initial core.
Even if the technique described in Japanese Patent Laid-open print No. 61-165682 is used, a sufficient effect is not achieved. In this technology, because the number of the high enrichment fuel assemblies of the initial core is at most about 20-30% of the total number of the fuel assemblies, there is a limit to the increase of the average enrichment of the initial core. Consequently, the effect for fuel improvement is not sufficient.